The invention relates to methods and apparatus for cleaning articles using supercritical and/or near-supercritical fluids. In particular, the present invention relates to using differences in contaminant solubility and solvent density at various temperatures and/or pressures to effect cleaning action, to influence solvent and/or contaminant movement in cleaning apparatus, and to facilitate concentration of contaminants within cleaning apparatus and their subsequent removal.
Cleaning Using Solvent Action
It has long been known to use solvents in removing organic and inorganic contaminants from art articles. In such processes, the contaminated article to be cleaned is contacted with the solvent to solubilize and remove the contaminant. In a vapor degreaser, subsequent evaporation of the solvent separates the solvent and the contaminant, and the solvent vapors are redirected to the article to further clean it. The contaminant is typically concentrated in the evaporation step, being removed as a precipitate, a separate liquid phase, or as a concentrated solution in the original solvent.
An example of the above process is described in U.S. Pat. No. 1,875,937, issued Sep. 6, 1932 to Savage. Grease is removed from the surface of metal castings and other nonabsorbent bodies by means of solvents, while contaminants collect in the bottom of the apparatus and are drawn off from time to time through a valve.
One of the drawbacks of this type of cleaning process is that the cooling surfaces also have a tendency to condense water out of the atmosphere in addition to cooling and condensing the solvent. This condensed water then becomes associated with the solvent and thus comes into contact with the metal parts of the cleaning apparatus and with the article being cleaned.
U.S. Pat. No. 2,123,439, issued Jul. 12, 1938, to Savage, describes how this problem of condensing water with the solvent may be overcome by first contacting the atmosphere with condensing surfaces at a temperature above the dew point of the atmosphere in which the operation is being carried out, but substantially below the condensing temperature of the solvent. The condensed solvent is drawn off for use in the cleaning process, while the remaining vapors are brought into contact with still cooler surfaces (cooler than the dew point) to condense out the water so it can be removed.
An alternative to the above process of condensing the solvent on a cold surface and then contacting the article to be cleaned with condensed solvent is to cool the article itself. For example, U.S. Pat. No. 3,663,293, issued May 16, 1972, to Surprenant et al., describes how the degreasing of metal parts may be accomplished by generating vapors of a solvent from a liquid sump, establishing a desired level of solvent vapor by adjusting the temperature of condensing means, and introducing a contaminated cold article into the solvent vapors, thereby causing the vapor to condense on the article. Condensate containing the contaminant falls from the article into the sump, and the article is removed from the solvent vapor when its temperature reaches the solvent vapor temperature (thus precluding further solvent condensation on the article).
Cleaning Using Supercritical Fluids
In an effort to improve on vapor degreasing methods, supercritical (and near-supercritical) fluids have been used as solvents to clean contaminants from articles. NASA Tech Brief MFS-29611 (December 1990), describes the use of supercritical CO2 as an alternative for hydrocarbon solvents conventionally used for washing organic and inorganic contaminants from the surfaces of metal parts.
A typical supercritical fluid cleaning process involves contacting the part to be cleaned with a supercritical fluid. The supercritical fluid, having solubilized contaminants and thus removing them from the part, then flows to a zone of lower pressure through an expansion valve. This depressurization causes the solvent fluid""s state to change from supercritical to subcritical, resulting in separation of the solute (that is, the contaminant) from the solvent. Relieved of its burden of contaminant, the cleaned solvent fluid is then compressed back to a supercritical state and again brought into contact with the part if further cleaning is desired.
A different approach to cleaning with supercritical fluids is described in U.S. Pat. No. 4,944,837, issued Jul. 31, 1990 to Nishikawa et al. The method is applied to cleaning a silicon wafer in an atmosphere of supercritical carbon dioxide which contacts the wafer to solubilize the contaminant. After cleaning is complete, carbon dioxide is cooled to below its supercritical temperature (i.e., the system pressure is reduced and the carbon dioxide attains equilibrium between the liquid and gas phases) before removal of the cleaned wafer from the apparatus.
While effective, these processes are relatively inefficient because of the energy consumed in each pressurizaton-depressurization cycle. Further energy losses and increases in equipment complexity are associated with moving the solvent through the apparatus in both supercritical and subcritical states.
The present invention (an improved cleaner using supercritical and/or near-supercritical fluids) includes an apparatus which avoids or reduces several of the shortcomings noted above by keeping the solvent fluid in a supercritical or near-supercritical state in a pressure vessel throughout the cleaning and contaminant removal process. The pressure vessel comprises sealable access means to the vessel interior such as a door, lid, pressure lock, hatch, valve, etc. Note that a pressure lock may itself comprise a pressure vessel. Sealable access means may also comprise ports to introduce and/or remove articles to be cleaned, to remove (and if desired, recover) concentrated contaminants (including contaminated solvents), and to replenish the solvent as needed. Note that cosolvents and/or adjuvants which may be present as components in a solvent fluid may or may not also be in a supercritical state during normal operation of the cleaner.
Solubilized contaminants are concentrated and recovered through use of heating and/or cooling means within the pressure vessel which cause temperature changes in a solvent fluid which change contaminant solubility in the fluid. Even during contaminant recovery in the above improved cleaner, however, the solvent fluid remains in a supercritical or near-supercritical state. Consequently, the energy consumption is reduced (and efficiency is increased) over existing cleaners in which the solvent must be heated to account for enthalpy losses upon depressurization and compression to recycle the solvent and use it in the supercritical state.
In preferred embodiments of the improved cleaner, mechanical pumps are virtually unnecessary (initial pressurization and replacement of solvent fluid during operation can simply be accomplished by heating liquid carbon dioxide) because bulk-flow and micro-flow convection currents provide the desired fluid circulation. Additionally, because of the large density changes with low temperature differences and the low viscosity, supercritical fluids can move very quickly in response to relatively small temperature differences in different fluid zones. Such rapid solvent fluid movements, however, are detrimental to creating relatively large temperature differences within the solvent fluid necessary to effect large solubility differences within the supercritical fluid thereby diminishing the internal cleaning/recycling functionality of the invention. The rapid movement of the supercritical fluid past a heat exchanger surface reduces the amount of heat transfer; greater temperature differentials between the fluid and heat exchanger surface aggravates the problem. A solution is to increase the effective area for heat transfer by providing more contact time with the supercritical fluid by altering the fluid flow patterns through use of insulated baffle means.
In certain preferred embodiments, heat pumps may be used to maintain a desired temperature differential between heating zones (containing, for example, one or more heating means) which are spaced apart from cooling zones (containing, for example, one or more cooling means). In such cases, the cooling means would comprise, for example, the heat pump evaporator coils, while heating means would comprise, for example, the heat pump condenser coils. Auxiliary heating and cooling will be needed since 100% thermal efficiency cannot be achieved. Heating and cooling means may also include passive radiators thermally coupled to ambient fluids such as air (the stainless steel pressure vessel walls conduct large quantities of heat from the supercritical fluid necessitating insulation of the hot zone to achieve improved temperature control). Thermoelectric devices such as resistance heaters (for heating) and Peltier devices (for heating and/or cooling) have been successfully employed in the experimental operation of this invention. Peltier devices in particular may be employed to establish or augment a desired temperature difference across a baffle, thus providing a functional equivalent of insulated baffle means. For purposes of the present invention, insulated baffle means comprise such combinations of Peltier devices and baffles. Hence, convective fluid flow in improved cleaners of the present invention may be easily reversed in whole or in part by reversal of current flow in one or more Peltier junctions within the pressure vessel provided the proper configuration for exploiting the gravitational forces is used; such real-time modulations may be beneficial for localized supercritical fluid currents to dislodge, relocate, or separate contaminants from the part and out of the solvent.
Control of either bulk or micro convective fluid movements in the above improved cleaner is preferably facilitated by insulated baffle means (to direct or channel the fluid stream flow). Insulated baffle means generally separate portions of moving fluid streams from portions of other moving fluid streams, wherein a temperature difference exists between the separated portions. The baffle insulation should be such that the heat transfer by conduction across the baffle is much less than the heat transfer by convection of the supercritical fluid moving between the hot and cold zones. This criterion is necessary to encourage the desired mass transfer (i.e., means to move clean supercritical fluid to the part and contaminants from the part) while also providing a large temperature difference between fluid zones to effect separation of the contaminant from the supercritical fluid. Note that insulated baffle means separate only portions of fluid streams. That is, fluid stream separation is not total but merely sufficient to maintain a desired temperature difference between portions of (preferably at least partly supercritical) solvent fluid streams to facilitate convective fluid flow and/or to achieve or maintain desired conditions of solubility or insolubility of one or more contaminants in a solvent fluid.
Insulated baffle means of the above improved cleaner comprise at least one space-occupying rigid or semi-rigid baffle structure which in use separates portions of at least two moving fluid streams comprising supercritical and/or near supercritical fluid, wherein a temperature difference exists between portions of at least two of the separated fluid stream. In practice, insulated baffle means can comprise, for example, structures having substantially planar and/or at least partially curved external surfaces and incorporating one or more evacuated spaces and/or other thermal insulators substantially in a thermal path between the external surfaces (and/or portions thereof) to restrict convective heat transfer so that discrete temperature (and hence solubility) zones may form in the fluid. The thermal insulators may comprise, for example, rubber, plastic and/or fibrous materials having low thermal conductivity relative to solvent fluids intended for use.
Insulated baffle means is primarily designed to provide the necessary temperature difference between fluid zones for effective cleaning and solvent replenishing. The insulated baffle is also used to enhance cleaning action by, for example, directing the convective flow of a stream of relatively clean solvent fluid to an article to be cleaned, possibly increasing flow velocity by decreasing stream cross-sectional area and/or by other means. Articles to be cleaned preferably rest on support means comprising stationary or adjustable shelves, or they may be rotated and/or translated during cleaning by support means which comprise a robotic manipulator. Note that the size and/or location of holes or ports in individual baffles and/or the size and configuration of gaps between baffles and/or between pressure vessel walls and baffles comprising insulated baffle means, as well as individual baffle surface contours and/or orientations with reset to a pressure vessel may be individually or collectively adjustable (as by closed loop control systems and/or by thermally active elements such as, for example, bimetallic elements analogous to those within a thermostat). Such adjustments may preferably be made, for example, to facilitate modification of convective fluid flow velocities and/or patterns, and/or contaminant dissolving power of solvent fluid, and/or contaminant separation from solvent fluid. Such baffle adjustments may be made in substantially real time to, for example, either accentuate or attenuate convective fluid flow characteristics to achieve, for example, improved cleaning action and/or improved contaminant concentration and/or recovery functions.
Static baffles are also useful for the economical and highly reliable operation of the cleaner. Different designs can provide cleaning performance benefits. For example, a baffle with only a center hole effects mass transfer through oscillating, pulsed flow in which the hot fluid surges through the hole, mixes rapidly with the cold fluid (decreasing the contaminant concentration in the cold fluid), and then the cold fluid surges into the hot zone with the cold fluid plume transferring the contaminant to the separation zone. Alternately, a baffle with an outer open ring and center hole permits hot fluid to flow though the outer ring and cold fluid downward through the center hole; thus causing first-in first-out mass transfer.
Insulated baffle means (whether adjustable or non-adjustable) may also be used to facilitate removal of contaminants from contaminated solvent fluid by, for example, directing the flow of a stream of solvent fluid containing one or more dissolved contaminants toward a heat source or sink (that is, heating means or cooling means, respectively) which will raise or lower the solvent fluid temperature sufficiently to cause the desired contaminant separation. Precipitated contaminants may, in turn, be allowed to settle out of the stream by increasing stream cross-sectional area and slowing stream velocity, or they may be superconcentrated using, for example, a screen separator, demister, impinger, separatory funnel, maze of tortuous return flow channels, or cyclone as the stream is directed to travel a curved path by insulated baffle means. These devices could be mounted directly to the baffle, for example in the first-in, first-out baffle configuration a mechanical filter could be mounted to the ring opening to collect particulates (for example, precipitated contaminant, inorganic materials, dust, or metal shavings) or coalescing liquid contaminant droplets before returning the clean hot fluid to the cold zone. Another configuration is to have a side piping to the main cleaning chamber in which a heat exchanger is located. The supercritical fluid would move through this side piping via natural convection currents. The filters, impingers, or cyclone could be located within this piping to help segregate the contaminants from the supercritical fluid (much like the behavior of a steam trap in a pipe flowing steam). Contaminants which have been concentrated by separation and/or those which have been superconcentrated by one or more of the above methods are intermittently or continuously removed from the cleaning apparatus via recovery means (such as, for example, a sump drain valve or a pressure lock for removing semisolid contaminants) positioned within a pressure vessel port to recover the contaminants.
Thus, preferred embodiments of the invention include an apparatus for removing contaminants from an article to be cleaned, the apparatus comprising a pressure vessel and support means within the pressure vessel for supporting the article to be cleaned. Heating means within the pure vessel facilitate convective flow of a solvent fluid within the pressure vessel, and cooling means within the pressure vessel (which are spaced apart from the heating means) also facilitate convective flow of a solvent fluid within the pressure vessel. Finally, insulated baffle means within the pressure vessel are positioned between the heating means and the cooling means for maintaining at least one temperature difference between zones in a solvent fluid within the pressure vessel.
Note that first and second heating means (or a plurality of heating means) spaced apart within the pressure vessel, and/or first and second cooling means (or a plurality of cooling means) spaced apart within the pressure vessel and apart from the heating means, may also be used to facilitate convective flow of a solvent fluid within the pressure vessel. Note also that heating and/or cooling means within the pressure vessel and spaced apart from any other heating or cooling means may be used to facilitate separation of contaminants from a fluid within the pressure vessel. In certain embodiments of the improved cleaner, heating means and/or cooling means may serve the dual functions of facilitating both convective fluid flow and separation of contaminants from a solvent fluid.
In any of the above embodiments of the present invention, the insulated baffle means may comprise at least one insulated baffle having an annular gap and a substantially centered hole, and/or at least one insulated baffle having a peripheral hole. Insulated baffle means may also comprise at least one adjustable baffle hole. An improved cleaner may also comprise a fluid within the pressure vessel, the fluid comprising, for example, one or more supercritical and/or a near-supercritical fluids.
The invention also includes a method of facilitating fluid flow within a pressure vessel. The method comprises heating a first portion of the fluid with heating means and cooling a second portion of the fluid with cooling means. A portion of the heated first fluid portion is separated from a portion of the cooled second fluid portion with insulated baffle means for maintaining at least one temperature difference between fluid zones within the pressure vessel to facilitate convective fluid flow within the pressure vessel. The invention further includes a method of directing fluid flow within a pressure vessel, the method comprising the above steps followed by directing at least a portion of the convective fluid flow within the pressure vessel using insulated baffle means. The fluid referred to in these methods may of course comprise one or more supercritical and/or near-supercritical fluids.
Another method included in the present invention is a method of removing contaminants from an article to be cleaned using a solvent fluid within a pressure vessel. The method comprises supporting the article to be cleaned with support means within the pressure vessel, heating a first portion of the fluid within the pressure vessel with heating means, and cooling a second portion of the fluid within the pressure vessel with cooling means. A portion of the heated first fluid portion is separated from a portion of the cooled second fluid portion with insulated baffle means within the pressure vessel for maintaining at least one temperature difference between zones in the fluid to facilitate convective fluid flow within the pressure vessel. And insulated baffle means direct at least a portion of the convective fluid flow toward the article to be cleaned to remove contaminants from the article.
Another method of the present invention is that for concentrating contaminants removed from an article to be cleaned using a fluid within a pressure vessel. The method comprises removing contaminants from the article to be cleaned by the above method and then concentrating by separation in the convective fluid flow at least a portion of the removed contaminants from the fluid by heating or cooling the fluid at a location within the pressure vessel and spaced apart from the article to be cleaned. Contaminants removed from an article and concentrated as above may be superconcentrated within a pressure vessel. Methods to accomplish this comprise directing by insulated baffle means at least a portion of the convective fluid flow comprising precipitated contaminants toward separation means comprising, for example, a separatory funnel, a screen separator and/or a cyclone separator within the pressure vessel and superconcentrating at least a portion of the precipitated contaminants by separation within the separatory funnel, the screen separator and/or the cyclone separator.